Currently, SEC represents the most widespread method of determining molecular weight and molecular weight distribution of polymers. The method capitalizes on the difference in size of molecular species and their ability to penetrate into pores of the column packing material. Large species cannot enter smaller pores and, therefore, move with the mobile phase along the column with a higher velocity than that of smaller species which explore both large and small pores incorporating stagnant zones of the mobile phase. Though the theory that relates the size of macromolecular coils to diameters of accessible pores and the accessible portion of the total pore volume of the packing material, is far from being mature, the practical usefulness of SEC in the macromolecular research is beyond any doubt. However, SEC of polymers represents an analytical technique, employing injection into the column of a small portion of a diluted polymer solution, only, followed by the elution of the probe with the mobile phase. This precondition provides the independency of macromolecular coils from each other, and prevents any competition between them for space available to the mobile phase in the chromatographic column.
Thus far, only micro-preparative separations of macromolecular species that strongly differ in their size have been described, for instance, SEC separation of nanosponges from their clusters. Davankov V. A., Timofeeva G. I., Ilyin M. M., Tsyurupa M. P. (1997) J. Polymer Sci., Part A: Polymer Chem. 35: 3847-3852. On a larger scale, SEC (so-called gel filtration) operates in the purification of proteins from inorganic salts, the former being excluded from a hydrogel-type packing, while small inorganic molecules enter the gel phase and reside longer in the column.
Separation of inorganic ions by analytical-scale SEC is also well documented, mainly by publications of Yoza and co-workers, on densely crosslinked hydrogels, such as Sephadex G-15. Egan B. Z. (1968) J. Chromatogr. 34: 382; Yoza N., Ohashi Sh. (1969) J. Chromatogr. 41: 429-437; Ueno Y., Yoza N., Ohashi Sh. (1970) J. Chromatogr. 52: 321-327; Yoza N., Ogata T., Ohashi Sh. (1970) J. Chromatogr. 52: 329-338; Ogata T., Yoza N., Ohashi Sh (1970) J. Chromatogr. 58: 267-276; Yoza N. (1973), J. Chromatogr. 86: 325-349. Though the elution volumes of different hydrated ions were found to basically correlate with their size, the situation was often complicated by numerous side effects. Thus, interaction of nitrate or chlorate anions with the sorbent matrix or interaction of barium cations with the hydroxyl groups of the matrix cause retention of both these ions and their oppositely-charged partner ions, which results in the rise of the retention volumes of electrolytes over the value of the hold-up volume of the column. To date, no attempts have been made to apply SEC on neutral packings for preparative separations of electrolytes.
An entirely different area of research is represented by analytical-scale separations of diluted solutions of polymeric inorganic compounds on hydrogels, like Sephadex G-15 to G-50 (crosslinked dextran) or Bio-Gel A (crosslinked agarose). From the very beginning, these materials have been designed for gel permeation chromatography, so that size exclusion mechanisms of separation of inorganic polymers, as condensed phosphates, molybdophosphates, polymeric silicic acid and the like have been early recognized. A detailed review by Yoza (N. Yoza, J. Chromatogr., 86 (1973) 325-349, Gel chromatography of inorganic compounds) also deals with additional effects that complicate the SEC separation mechanism, such as adsorption, ion exclusion, and secondary complexation equilibria in solution. As for simple metals and anions, a correlation has been observed between the retention and the radii of hydrated ions. N. Yoza, S. Ohashi, J. Chromatogr., 41 (1969) 429-437, Chromatographic behavior of alkaline earth metal ions on Sephadex G-15 columns; J. Porath, Metab. Clin. Exp., 13 (1964) 1004; Y. Ueno, N. Yoza, S. Ohashi, J. Chromatogr., 52 (1970) 321.
Rona and Schmuckler eluted Dead Sea concentrated brine on a Bio Gel P-2 (crosslinked polyacrylamide) column with the result of obtaining a lithium-enriched fraction free from calcium and magnesium. M. Rona, G. Schmuckler, Talanta, 20 (1973) 273-240, Separation of lithium from Dead Sea brines by gel permeation chromatography. Bio Gel P, however, is known to retain cations and probably enter hydrogen-bond interactions between the anions and the amide hydrogen. The elution order of chlorides was thus different from one expected for size exclusion mechanisms, namely, K+, Na+, Li+, Mg2+, and Ca2+, all emerging before the hold-up (dead) volume.
Processing of aqueous solutions of mineral electrolytes is the field of application of ion exchange chromatography. Cation and anion exchange resins readily retain, selectively or non-selectively, cations or anions from the initial solutions, thus leaving acids or bases, respectively, in the filtrate. To remove adsorbed ions from the resin and regenerate the used ion exchanger bed, appreciable amounts of acids or bases are needed. Moreover, all regeneration processes produce large volumes of strongly mineralized waste solutions that are expensive to neutralize and dispose.
To avoid the waste disposal problems, several reagent-free separation processes have been developed. One of them is based on the fact that the sorption selectivity of a resin may vary significantly with the temperature of the column. Muraviev & Hamizov, in Ion Exchange Technology for Today and Tomorrow, Proceedings of IEX2004, Ed. M. Cox, Cambridge, July 2004, 151-160, Sustainable development and ion exchange: Green ion exchange technologies. A desired component of a mixture of electrolytes can thus be preferably adsorbed by the ion exchanger at a low temperature and then released at an enhanced temperature. The method, however, requires cyclic heating and cooling of large columns, which is combined with energy consumption and furthermore reduces the life expectancy of the resin.
Other reagent free processes of separating electrolytes are based on phenomena of retardation of electrolytes by special ion exchangers. As early as 1950, A. B. Davankov et al. applied for and, five years later, received a USSR patent (Davankov et al., USSR Patent 100692 (1955)) for “A method of removing salts from aqueous solutions by using amphoteric ion exchange resins.” For the first time, the polymeric material incorporated both cationic and anionic functional groups and, therefore, simultaneously retained both anions and cations of a dissolved salt. A real success in the removal and/or separation of electrolytes by that “ion retardation” process was achieved with amphoteric “snake-cage polyelectrolytes” suggested 1957 by Hatch, Dillon and Smith (M. J. Hatch, J. A. Dillon, H. B. Smith, Ind. & Eng. Chem., 49 (1957) 1812-1819, Preparation and use of snake-cage polyelectrolytes) and defined as a “cross-linked polymer system containing physically trapped linear polymer.” These materials have been commercialized by Dow Chemical Co. under the name “Retardion.” Dow Chemical Co., Midland, Mich., Tech. Service Bull. 164-62, “Ion Retardation.” A typical snake-cage amphoteric polyelectrolyte, Retardion-550WQ2, was made by polymerizing ar-vinylbenzyl trimethyl ammonium chloride inside sulphonated cation exchanger Dowex-50W×2 and had strong basic and acidic functional groups. On a 100 ml column packed with this resin, a 20 ml sample of 2.0 M in NH4NO3 and 1.6 M in HNO3 was almost completely separated into constituents. M. J. Hatch, J. A. Dillon, Ind.&Eng. Chem., Process. Design and Development, 2/4 (1963) 253-263, Acid Retardation. A Simple Physical Method for Separation of Strong Acids from Their Salts.
Similarly, 0.49 M FeCl2 partially separated from 3.15 M HCl. M. J. Hatch, J. A. Dillon, Ind.&Eng. Chem., Process. Design and Development, 2/4 (1963) 253-263, Acid Retardation. A Simple Physical Method for Separation of Strong Acids from Their Salts. Other examples of “snake-cage polyelectrolytes” (M. J. Hatch, J. A. Dillon, H. B. Smith, Ind.&Eng. Chem., 49 (1957) 1812-1819, Preparation and use of snake-cage polyelectrolytes) are polyvinylpyridine entrapped in the network of a sulphonated polystyrene-divinylbenzene (PS-DVB) copolymer Dowex-50, or polyacrylic acid entrapped in the matrix of a strong basic PS-DVB anion exchanger Dowex-1. In general, ion retardation, a column absorption method, produced effective industrial separations of aqueous mixtures of strong electrolytes.
In 1958, in a theoretical study on activity coefficients of electrolytes in the resin phase, Nelson and Kraus arrived at a conclusion that, “because of the relatively low activity coefficients of HCl in the examined anion exchanger Dowex-1×10,” separation of HCl from concentrated halide solutions (LiCl, MgCl2) is possible. F. Nelson, K. A. Kraus, J. Am. Chem. Soc., 80 (1958) 4154-4161, Anion-exchange studies. XXIII. Activity coefficients of some electrolytes in the resin phase.
Hatch and Dillon re-discovered 5 years later that conventional ion exchange resins efficiently separate concentrated acids from their salts under conditions that exclude normal ion exchange. M. J. Hatch, J. A. Dillon, Ind.&Eng. Chem., Process. Design and Development, 2/4 (1963) 253-263, Acid Retardation. A Simple Physical Method for Separation of Strong Acids from Their Salts. This finding contradicted the concept of “ion exclusion” according to which all strong electrolytes should be effectively excluded from absorption into ion exchange resins, because of the Donnan equilibrium effect. F. Helfferich, Ion Exchange, 1962, McGraw-Hill, N.Y., p. 134; R. M. Wheaton, W. C. Bauman, Ind.&Eng. Chem., 45 (1953) 228. The strong basic anion exchange resin Dowex-1×8 was found to function especially well, and the authors introduced a new term, “acid retardation.” “Such separations can be defined as “acid retardation” separations, since they are based on a preferential absorption of strong acids, which causes the movement of the acid on the bed to be retarded—i.e., slowed down—relative to the movement of the salt.” Acid retardation, like ion retardation can be done at high flow rates, especially at elevated temperatures. These processes have been optimized and since 1976 widely exploited by Eco-Tec, Canada on the industrial scale. C. J. Brown, V. Sheedy, M. Palaologou, R. Thompson, Proceedings of Annual meeting of minerals, metals, materials society, Orlando, Fla., USA, 1997, TP126. Interestingly, no satisfactory theoretical explanations of the acid absorption could be provided during the 40 years that passed after the appearance of the above classical works.
Recently, it has been suggested that the mechanisms of industrially important processes of “ion retardation” and “acid retardation” on amphoteric and anion-exchange resins, respectively, have much in common with SEC separations; and a new type of preparative chromatographic process has been described—namely, separation of simplest mineral electrolytes by means of frontal SEC of their aqueous concentrated solutions on neutral microporous materials. Tsyurupa M. P., Davankov V. A. (2004) Doklady Akad. Nauk RAN 398/2: 198-200; Davankov V. A., Tsyurupa M. P. (2005) J. Chromatogr. A, 1087: 3-12.
The size of hydrated mineral cations and anions is relatively small, on the level of several angstroms, and so their SEC separation requires using microporous stationary phases. Microporous neutral hypercrosslinked polystyrene sorbents proved to be very promising. These materials represent the first and up to now the only microporous non-functionalized polymeric adsorbing material with a pore size comparable to diameters of hydrated electrolyte ions. Several types of neutral hypercrosslinked polystyrene sorbents are currently manufactured by Purolite International (Pontyclun, UK) on an industrial scale. Hypersol-Macronet™ Sorbent Resins, Purolite Technical Bulletin, The Purolite Company, UK (1995). Another useful column packing material is microporous activated carbons prepared by pyrolysis of hypercrosslinked polystyrene sorbent beads. Aleksienko N. N., Pastukhov A. V., Davankov V. A., Belyakova L. D., Voloshchuk A. M. (2004) Russ. J Phys. Chem. 78/12: 1992-1998. Due to high rigidity of the framework of both the carbons and hypercrosslinked polystyrene, their largely hydrophobic micropores avoid collapsing and accommodate water that can be accessed by small molecules and ions.
Davankov et al. dealt with some distinguishing features of SEC that are most important for a preparative-scale process, such as:                species separated by an exclusion chromatography process are transported along the column by the mobile phase, but move faster than that mobile phase;        a size-exclusion column, being equilibrated with the mixture under separation, always incorporates a liquid the concentration of which remains reduced with respect to the excluded species;        concentration of all species that appear in the corresponding fractions of the effluent in a frontal size-exclusion chromatography rises again to the level of their concentration in the initial mixture, or, in other words, frontal SEC does not cause any dilution of solutes; and        rather the opposite, separation of a concentrated two-component mixture by SEC is connected with a self-concentrating effect of the components in the corresponding fractions of the effluent, according to inherent results of an “ideal separation process.” Davankov V. A., Tsyurupa M. P. (2005) J. Chromatogr. A, 1087: 3-12.        
Basic inherent drawbacks of all processes where one or all components under separation are retained by the sorbent is the fact that the elution of the retained components from the sorbent requires additional amounts of the eluent and thus results in an unavoidable dilution of the initial mixture with the respect to retained components. This is not the case with exclusion chromatography processes because excluded components do not depart from the mobile phase and emerge from the column in non-diluted state.
Another fundamental drawback of using hydrophilic adsorbing materials, like ion exchange resins, in practical separations of electrolytes is the fact that the resin beds change their volume with the concentration and pH of changing solutions. This shortens the lifetime of materials and significantly complicates the operation of large columns. Operating large units in industrial processes is beneficial compared to many cycles run on a smaller column. Mixing and dilution of chromatographic zones in exclusion chromatography take place on borders of the zones only. Therefore, the more cycles and borders, the more mixed and diluted the fractions.